An instrument known as an integrating nephelometer has been in existence for many years for measuring the amount of light scattered by particulates (aerosols) in the air. Several commercial integrating nephelometers are available from companies such as TSI Inc. (Minnesota), Radiance Research (Washington), and Optec Inc. (Michigan). The instruments are sold for monitoring air pollution, for atmospheric and climate research, and for monitoring clean rooms. These instruments are called integrating nephelometers because they measure the total light scattered at all angles altogether. Much information is lost about the particulates because their size, shape and composition influence the angles at which the light is scattered.
A polar nephelometer (as in a mathematical polar plot using angle and radius) is able to measure the light scattered at individual angles. As will be discussed in more detail below, there have been several research instruments built by different research groups, using multiple detectors or moveable arm-mounted detectors. There are no known commercial instruments developed from such research devices, as they are relatively complex and expensive to build, and also difficult to use for field implementations. Thus it remains a requirement in the art to provide a simpler design, higher sensitivity, and higher resolution that these Prior Art research devices, and provide them in an apparatus that can be readily commercialized.
The following two publications detail the technique of creating and analyzing a wide-angle camera image of a laser beam to measure particulates: Barnes, J. E.; Bronner, S.; Beck, R.; Parikh, N. C.; Boundary layer scattering measurements with a CCD camera lidar, Applied Optics, 42, 2647-2652, 2003, and Barnes, John E., N. C. Parikh Sharma and Trevor B. Kaplan, Atmospheric aerosol profiling with a bistatic imaging lidar system, Applied Optics, 46, 2922-2929, May, 2007, both of which are incorporated herein by reference.
There are many publications of research polar nephelometers using multiple detectors, single detectors on movable arms or variations of these. The following are three examples: Kaller, W., A new polar nephelometer for measurement of atmospheric aerosols, J. of Quantitative Spectroscopy & Radiative Transfer, 87, 107-117 (2004), Barkey, B. and K. N. Liou, Polar nephelometer for light-scattering measurements of ice crystals, Optics Letter, 26, 232-234 (2001), Gayet, J. F., O. Crepel, J. F. Fournol and S. Oshchephov, A new airborne polar Nephelometer for the measurements of optical and microphysical cloud properties. Part I: Theoretical design, Ann. Geophysicae, 15, 451-459 (1997), all of which are incorporated herein by reference.
Hill et al., U.S. Pat. No. 7,126,687, issued Oct. 24, 2006 and incorporated herein by reference, discloses a method and apparatus for determining absorption and morphology of individual airborne particles. In particular, the Hill device uses a single probe laser and a number of trigger lasers in combination with a plurality of detectors to measure two-dimensional optical scattering at different wavelengths.
Wang et al., Published U.S. Patent Application No. 2009/0039249, published Feb. 12, 2009, and incorporated herein by reference, discloses a size segregated aerosol mass concentration measurement device. Wang uses an integrating signal conditioner to integrate the electronic signal from his detector.
In general, the polar nephelometers of the Prior Art have complicated designs which are also are not very sensitive. They use multiple detectors or rotating mirrors to get the angular information instead of creating an image. Some are able to measure individual particles, which can be an advantage for some scientific work. But such instruments may not be suitable for the commercial integrating nephelometer market. A need still exists in the art for a simplified imaging polar nephelometer, which can measure average light scattered from all the air and particles in a chamber.